The leukotrienes constitute a group of locally acting hormones, produced in living systems from arachidonic acid. The major leukotrienes are Leukotriene B.sub.4 (abbreviated at LTB.sub.4), LTC.sub.4, LTD.sub.4, and LTE.sub.4. The biosynthesis of these leukotrienes begins with the action of the enzyme 5-lipoxygenase on arachidonic acid to produce the epoxide known as Leukotriene A.sub.4 (LTA.sub.4), which is converted to the other leukotrienes by subsequent enzymatic steps. Further details of the biosynthesis as well as the metabolism of the leukotrienes are to be found in the book Leukotrienes and Lipoxygenases, ed. J. Rokach, Elsevier, Amsterdam (1989). The actions of the leukotrienes in living systems and their contribution to various diseases states are also discussed in the book by Rokach.
Three ICI European Patent Applications (EP 385,662; 462,812; and 462,830) disclose the compounds shown below as being inhibitors of the 5-lipoxygenase enzyme. Among other differences, these compounds differ from those of the present invention in that the nature of the linking unit, A.sup.1 --X.sup.1 or A--X, is consistently different from X.sup.3 in the present compounds. ##STR2##